1. Field of the Invention
The present invention relates to a projection objective for imaging a pattern provided in an object plane of the projection objective onto an image plane of the projection objective. The projection objective may be used for microlithography projection exposure machines. The invention relates, in particular, to exposure machines for semiconductor structures which are designed for immersion operation, that is to say in an aperture range where the image side numerical aperture NA is greater than 1.0.
2. Description of the Related Art
In the case of reducing optical imaging, in particular of projection lithography, the image side numerical aperture NA is limited by the refractive index of the surrounding medium in image space. In immersion lithography the theoretically possible numerical aperture NA is limited by the refractive index of the immersion medium. The immersion medium can be a liquid or a solid. Solid immersion is also spoken of in the latter case.
However, for practical reasons the aperture should not come arbitrarily close to the refractive index of the last medium (i.e. the medium closest to the image), since the propagation angles then become very large relative to the optical axis. It has proven to be practical for the aperture not substantially to exceed approximately 95% of the refractive index of the last medium of the image side. This corresponds to propagation angles of approximately 72° relative to the optical axis. For 193 nm, this corresponds to a numerical aperture of NA=1.35 in the case of water (nH2O=1.43) as immersion medium.
With liquids whose refractive index is higher than that of the material of the last lens, or in the case of solid immersion, the material of the last lens element (i.e. the last optical element of the projection objective adjacent to the image) acts as a limitation if the design of the last end surface (exit surface of the projection objective) is to be planar or only weakly curved. The planar design is advantageous, for example, for measuring the distance between wafer and objective, for hydrodynamic behaviour of the immersion medium between the wafer to be exposed and the last objective surface, and for their cleaning. The last end surface must be of planar design for solid immersion, in particular, in order to expose the wafer, which is likewise planar.
For DUV (operating wavelength of 248 nm or 193 nm), the materials normally used for the last lens are fused silica (synthetic quartz glass, SiO2) with a refractive index of nSiO2=1.56 or CaF2 with a refractive index of nCaF2=1.50. The synthetic quartz glass material will also be referred to simply as “quartz” in the following. Because of the high radiation load in the last lens elements, at 193 nm calcium fluoride is preferred for the last lens, in particular, since synthetic quartz glass would be damaged in the long term by the radiation load. This results in a numerical aperture of approximately 1.425 (95% of n=1.5) which can be achieved. If the disadvantage of the radiation damage is accepted, quartz glass still allows numerical apertures of 1.48 (corresponding to approximately 95% of the refractive index of quartz at 193 nm). The relationships are similar at 248 nm.